Severe Weather Headed Our Way

PHILADELPHIA (CBS) – A strong cold front has been wreaking havoc on the weather across the US this week, and now it’s our turn!

This system separates warm, moist air to the east from much cooler, drier air to the west, and that clash of air masses is producing a volatile situation as the front marches eastward tonight.

Thunderstorms are moving from southwest to northeast and there is a good amount of shear in the atmosphere. For tornadoes to form, you need pretty specific conditions, and wind shear plays a large part in that.

There are two types: speed shear (wind speed increasing as you move up through the atmosphere), and directional shear (wind direction changing as you move upward through the atmosphere). You must have both for a tornado to form.

In an environment like the one we’re seeing this evening, we have plenty of speed shear with the high-velocity winds aloft, and just the tiniest bit of directional change could prompt a thunderstorm cell to start rotating.

I want to stress that by far, the biggest impact from these storms will be gusty winds and heavy rain, but a Tornado Watch means that conditions are conducive to rotating storms and thus, we can’t rule out an isolated tornado, especially in areas off to our west where the watch is active.

Tornadoes are most often formed in single-cell storms, called supercells, rather than within a squall line. As of this afternoon, the storms appear to be forming a line, which to me would indicate a greater likelihood of straight-line winds. We’ll have to watch out for discrete cells to develop out ahead of this line or storms to break off – those will be the ones to watch for tornadic activity.

But keep in mind, a tornado watch doesn’t mean a tornado is going to happen! If a warning is issued, that means a tornado has been spotted and you should take cover immediately. As always, we will update instantly on TV, on cbsphilly.com, and via social media if a warning should be issued for our area.

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Chief Meteorologist Kate Bilo joined the Eyewitness News weather team on CBS 3 and The CW Philly in October, 2010.
Bilo, a native of Phoenixville, returned to the Delaware Valley from AccuWeather in State College, Pennsylvania where she had been...